Your son doesn't lead with his hips. He lifts his leg and lets it aggressively fly out, which means it opens too early which means he is losing a ton of velo and stride length.
It appears (I can't fully tell from this view) he aggressively lifts up his lead leg to gain momentum. Have him just lift it, this is hurting him. Once he lifts his leg more passively, get him to work on really sitting on that back leg, and having his HIPS lead instead of his leg. His hips should move forward and down at the same rate. This is what your son needs to work on, not getting MORE aggressive with the front leg like you said in another comment.
So have your son learn to lead with the hips, with them going forward and down at the same rate. His back leg should be bent, so that loads his hip and glutes; don't only load the quads. Work on this, then post more video for analysis. The beginning of the delivery is what causes the issues at the end, not just the end being wrong.
Now, because you asked, I will answer about his arm cocking at FFS
Here's your son:
And there's sonny gray. His arm should be slightly more cocked, HOWEVER it could be I didn't pause the vid properly with your son although doubtful.
I'd recommend you paste these two citations in google scholar and read them;
Rod Whiteley. – Baseball throwing mechanics as they relate to pathology and performance – A review. – Journal of Sports Science and Medicine (2007) 6, 1-20
Hirashima M, Yamane K, Nakamura Y, Ohtsuki T. – Kinetic chain of overarm throwing in terms of joint rotations revealed by induced acceleration analysis. – J Biomech. 2008 Sep 18;41(13):2874-83.